Molecular Weight Calculator: How to Find the Molar Mass of Any Compound

TL;DR

• Type any chemical formula into the molecular weight calculator to get the molar mass in g/mol instantly
• The calculator uses current 2021 IUPAC standard atomic weights
• It handles parentheses, hydrates, and multi-element compounds — for example Ca(OH)2 or CuSO4·5H2O
• Molar mass in g/mol is numerically the same as molecular weight in amu/Da
• Free, runs in your browser, no account needed

Every chemistry calculation that involves converting between grams and moles starts in the same place: the molar mass of your compound. The molecular weight calculator works out the molar mass of any chemical formula in seconds, so you can move on to the actual chemistry without getting stuck in subscript arithmetic.

How to Use the Calculator

Open the molecular weight calculator and type the molecular formula into the input field — for example H2O, NaCl, C6H12O6, or Ca(OH)2. The calculator reads each element symbol, looks up its standard atomic weight, multiplies by the subscript count, and sums the result. The output is the molar mass in grams per mole (g/mol). For glucose (C6H12O6), it returns 180.16 g/mol. No periodic table required.

When to Use This Calculator

Preparing lab solutions: To make a 1 M solution of sodium chloride (NaCl), you need to dissolve one mole — 58.44 g — in enough solvent to reach one litre. Calculate the molar mass here, then multiply by the number of moles your protocol requires.

Stoichiometry problems: Balancing reactions and converting between reactant masses and mole ratios both require the molar masses of each compound. Enter each formula separately and record the results before working through the stoichiometry.

Checking textbook values: Textbooks sometimes round atomic weights differently from current IUPAC values. Run the formula through the calculator to confirm the figure for submission-quality work.

Pharmacy and formulation work: Calculating concentrations of active pharmaceutical ingredients often requires precise molar mass values. The calculator uses current standard atomic weights rather than rounded approximations.

Student exam prep: A molar mass error early in a multi-step calculation compounds into a wrong final answer. Verify the value here before committing it to your working.

Frequently Asked Questions

What is the difference between molecular weight and molar mass? Molecular weight refers to the mass of a single molecule in atomic mass units (amu or Da). Molar mass is the mass of one mole of a substance in grams per mole (g/mol). Numerically they are identical — water has a molecular weight of 18.015 amu and a molar mass of 18.015 g/mol. The molecular weight calculator returns both under one calculation.

How do I calculate molecular weight manually? Find the atomic weight of each element in the formula from the periodic table. Multiply each atomic weight by the number of atoms of that element. Add all the results. For CO2: carbon (12.011) + 2 × oxygen (15.999) = 44.009 g/mol.

Does the calculator handle hydrates and complex ions? Yes. For copper(II) sulfate pentahydrate, enter CuSO4(H2O)5 and the calculator parses the full formula including the water of crystallisation.

What atomic weight values does the calculator use? It uses the 2021 IUPAC standard atomic weights — the accepted reference values for chemical calculations and the same values used in published academic periodic tables.

The molecular weight calculator is free and requires no sign-up. For other precise maths tools, the big number calculator handles very large arithmetic, and the long division calculator shows step-by-step division working.